Hydrogen is an important energy source as it reacts with oxygen to release energy with the only byproduct being water. Hydrogen is gaining importance as a non-carbon dioxide-based, renewable fuel, often referred to as a “clean fuel,” because it can be reacted with oxygen in hydrogen-consuming devices, such as a fuel cell or combustion engine, to produce energy and water. As a result, the use of hydrogen as a fuel effectively solves many environmental problems associated with the use of petroleum based fuels. Safe and efficient storage of hydrogen gas is, therefore, essential for many applications that can use hydrogen. In particular, minimizing volume and weight of the hydrogen storage are important for portable hydrogen production and power generation.
Several methods of storing hydrogen currently exist but are either inadequate or impractical for wide-spread consumer applications. For example, hydrogen can be stored in liquid form at very low temperatures. Liquid storage, however, provides a volumetric density of about 70 grams of hydrogen per liter and, thus, does not provide a sufficient amount of hydrogen for portable hydrogen and power generation. In addition, the energy consumed in liquefying hydrogen gas is about 30% of the energy available from the resulting hydrogen. Finally, liquid hydrogen is not safe or practical for most consumer applications.
Compounds that store hydrogen have also shown potential as high capacity hydrogen sources. However, such compounds may be limited by the amount of hydrogen they can store, and by their weight. Ammonia borane (H3NBH3) is a low molecular weight solid hydrogen storage material containing a significant percent of hydrogen (about 19.6 percent by weight). At room temperature, ammonia borane is a white crystalline solid, which is stable in both water and air. While the most efficient means of producing hydrogen from ammonia borane is by thermal decomposition, or thermolysis, there are several problems that make this process difficult to perform. Specifically, ammonia borane may begin to decompose at temperatures above 85° C., however, higher temperatures are needed to release a substantial amount of hydrogen contained in the ammonia borane. Although the overall process of thermally decomposing ammonia borane is exothermic, heat is required to initiate the reactions. Moreover, thermal decomposition of ammonia borane involves competing reactions that may result in formation of undesirable byproducts, such as borazine, which may poison conventional fuel cells during power generation. Thus, many challenges remain that must be overcome to provide efficient and practical use of ammonia borane.
Due to the challenges associated with thermal decomposition of ammonia borane, much of the present research has shifted to derivatives thereof, such as sodium amidoborane (NaNH2BH3) and sodium borohydride (NaBH4). Unfortunately, such derivatives have a substantially lower gravimetric hydrogen concentration in comparison to ammonia borane.
In view of the above, there is a need in the art for methods and systems for producing hydrogen for power generation and other applications.